A hydronic system circulator is always filled with system fluid (water). As soon as the pump moves water out of its discharge port, it is immediately replaced by an equal volume of water entering through its inlet port. Remember, these are closed systems that are always completely filled with water. If the system is not filled with water, either there is a leak or the operator has failed to check the water level before startup.
A circulator pump should never be run without any water in it. Doing so will damage the pump. Always check to be sure it is filled with water before startup.
Water enters the inlet port of the circulator and flows directly to the impeller located in the volute (see Figure 10-18). The impeller is a rotating wheel that creates the centrifugal force required to move the water through the piping. The pump drive shaft enters the back of the impeller and exits the front through an opening called an eye. The centrifugal force of the rotating impeller accelerates the velocity of the water, forcing it away from the eye and around the inside of the pump body. The water is directed toward a discharge port that is much smaller than the pump inlet port. Squeezing the water through this smaller discharge port converts the water velocity to pressure. This is the pressure head (or pump head) developed by the pump to overcome the friction (pressure drop) created by the water flowing through the piping, valves, and other components of a hydronic heating system.